What are the different components of a joist?
In our last video we talked about different types of structural joists, today we're going to spend some time breaking down the components of a joist.
IMPORTANT PART OF THE JOB
Why even talk about this? Not planning on becoming a structural engineer, Joe.
I get it, but a handful of these terms are going to become important when we're talking to people in the field or simply want to sound knowledgeable. Some of these specific terms come into play from a suppression standpoint in sprinkler design and have code implications for us, so it's important to start with the fundamentals and understand structurally what's going on and accurately describe what we look at in a building structure.
PARTS OF A JOIST
So let's look at a bar joist, or an open web steel joist. Same thing here.
We have three main components which are the top chord, the web and the bottom chord. If you don't remember anything else from today, remember that the chord is on the top and bottom and the web is what happens in between. for some reason I struggle to keep these straight, but my reminder to myself is that web starts with a W and we commonly see bar joists with web members that make a repeated W zigging and zagging along the length of the joist.
If you've never heard that before it's because I think I just made it up, but it is helpful for me to remember that the web is that middle portion of a joist. if your musical maybe you can come up with some mnemonic device about the term chord and let me know.
So main members are top chord, web and bottom chord.
CHORDS OF JOISTS
The top chord for an open web steel joist is often a pair of angle members, or double angles. With a load from above, the top chord of a joist will be under compression. The flanges, or the points of the chord that are furthest from the center, resist bending downward due to compression.
The top chord of a bar joist is often where we hang sprinkler pipe from. This is the best part of the joist to accept the load. It's where the joists are designed to accept the load. The strongest point of a joist is where the members of the web connect with the top chord. I believe this is casually called a panel point.
One side note here is that the most common direction to run branch pipe for sprinkler system is perpendicular to structure. Why? Because this makes it easier to hang pipe without having to trapeze hang every single piece of pipe. If you run pipe perpendicular to structure you have plenty of options for hanging pipe appropriately.
If the joist is capable of handling loads along the bottom chord, thin sprinkler pipe and other systems can be hung from the bottom chord. However this is not the preferred location to add loads onto a joist. If you have a project where hanging from the bottom chord is the preference, then check with the structural engineer for approval before hanging from that location. In general, I've personally found it easier just to hang from the top chord all the time than to do the legwork and push a structural engineer to evaluate hanging from the bottom chord just to have them reject it after wasting time.
The bottom chord of a joist also often in a double angle configuration. The flanged arms of the angle on the bottom are in tension. the bending of the joist poles at the ends of the bottom chord and pushes the joist downward which tries to elongate that bottom chord. we spoke earlier about how steel is very strong in both tension and compression, and that's what makes it so viable for use as a bar joist here.
WEB OF JOIST
So we have the top and bottom chord, now let's take a look at the web for a bar joist.
Webs could be made from rods, crimped angle webs, or angles welded to the outside of chords. where the webs are located are not always stipulated on structural plans. It's usually on the fabrication shop drawings by a steel contractor that would know where these web members are going to be located.
In an ideal world, all of the web members are aligned joist to joist. This is the cleanest and easiest way to fabricate a project, but unfortunately it's not always the case. When we are looking to run pipe through the open web portion of a joist, between the top and bottom chords, we're going to want to know if those web members are aligned. If they're not, it's very difficult to route pipe through the open web portion. We're going to get sprinklers that are too close to the web members which would create obstructions, and we also may need to jog back and forth just to avoid those web members.
For consultants I found that it's easy to try and specify that pipe needs to be within these open web members, but trying to show me a 21 foot stick of pipe into the web and avoid the web members and avoid sprinkler obstructions by being too close to those web members can be difficult to accomplish. Not impossible, but it can be difficult and takes some coordination.
WEB MEMBER OBSTRUCTIONS
In terms of sprinkler obstructions, it's the size of these web members that become important. For a typical standard spray sprinkler, the three times rule affects how far away a sprinkler needs to be from any of these web members. When we get into extended coverage in special application sprinklers, the three times rule turns into the four times rule.
In that case, the goal is to move a sprinkler further away from an obstruction so that water can get to both sides of the obstruction without having a significant shadow. If the width of a structural member that the sprinkler experiences is say 4 inches wide, then the three times rule says that that sprinkler deflector has to be located at least 12 inches away from this member, where the four times row would say that it has to be at least 16 inches away from the member. Search the annex material of NFPA 13 for the three times rule or the four times rule, and you'll navigate to the section that talks about these in a lot more detail.
For the web members of an open web joist, rods might not be very big or present obstruction challenges that we can't overcome, but welded angles can start to accrue some serious thickness that will affect our sprinkler layout. If all of our sprinklers have to be 16 inches away from those web members, then we start to incur some serious limitations on where the sprinklers can go.
When we're laying out Sprinklers in big warehouse areas or retail stores or grocery stores or open office areas, we can't just ignore the locations of these open web joists. I've had projects where the open web joists look innocent enough, but when I get to a job site after we've fabricated all of the pipe outlets and everything, and then there's a whole line of sprinklers that are too close to a joist, it causes some serious issues that could have been avoided. It would have been far better to end up with an extra sprinkler on each branch line than it was to have to cut and splice pipe or jog around with sprigs in order to get a layout to work after everything is fabricated in the field and hung. going slow and taking your time here with getting all of the details can go a very long way in making and open structure space a smooth experience for the installers.
If we're in the field inspecting a newly installed system, lookout for these web members and how close they are to the sprinkler. If something doesn't look right, question it, take some measurements, and check in with NFPA 13 for compliance. Now is the time to get it addressed and fixed before the system is put into service.
The other identifier that we come across with open web joists are their bridging members. Bridging members run perpendicular to a joist and prevent the entire series of joists from twisting or collapsing in the same direction like a House of Cards. bridging members can also cause issues with sprinkler obstructions or main routing if the mains are run parallel to the joists.
In wood construction, bridging could be in the form of solid blocks that could make running perpendicular difficult or requires some coordination to drill holes to route pipe.
In this video, I used a bar joist as an example for identifying all of the structural components, but a composite wood joist or a wood truss operate in much the same way. We have top chord, bottom chord and web members, and the principles around them and how we deal with them for pipe routing and sprinkler obstructions are very much in a similar vein.
As we continue this series we'll look at other structural components and how they affect our Fire Protection systems.
I'm Joe Meyer, this is MeyerFire University.
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